This application claims priority from European Patent Application Number EP01202147.3, filed Jun. 5, 2001.
1. Field of the Invention
The present invention relates to a method and apparatus for testing the accessibility of porous materials with regard to compounds comprising large molecules.
2. Prior Art
The accessibility of porous materials with regard to large, often high-molecular weight compounds is an important feature, for instance in catalysis. The accessibility of porous material can change substantially during its preparation and processing. For example, during preparation or processing of the material diffusion barriersxe2x80x94either internal (pore plugging, small pores, etc.) or external (e.g. skin formation)xe2x80x94can be formed, thereby decreasing the accessibility of the material. Important factors that influence the formation of these barriers are the properties of the constituting components (morphology, size, reactivity, crystallinity, etc.) and the applied conditions (pH, T, time, etc.). Diffusional barriers can be formed during the catalyst""s life time or regeneration, due to sintering, contamination with metals (e.g. Fe, V, Ca, Na) or coke, or the formation of a low-accessibility skin on the catalyst""s surface. The formation of diffusional barriers and, as a consequence, a reduction in accessibility can lead to a major decrease of activity in the various catalytic reaction systems.
With the formation of diffusional barriers via a low-accessibility skin on the outer surface of the porous materials the entrance to the pores is narrowed, while the total pore volume will hardly be affected. This skin will only restrict relatively large and rigid molecules from entering the pores. Therefore, common static techniques to study the pore size of porous materials, such as Hg-porosity measurements, nitrogen adsorption, etc., are not suitable for showing the presence of such a skin. In order to monitor skin formation and the accessibility of the pores with regard to large, often high-molecular weight compounds and to be able to develop catalysts with high accessibility, it is important that this accessibility can be measured fast, easily and accurately in a dynamic way. Moreover, it is a necessity that the test results can be correlated to the accessibility of the porous materials under conditions of use. In this specification, these conditions will be referred to as the application conditions.
A method for measuring the uptake of asphaltenes, i.e. Kuwait long residue, by hydrotreating catalysts is disclosed by J. Saint-Just, Ind. Eng. Chem. Prod. Dev. Vol. 19, 1980, pp. 71-75. This measurement is performed by circulating an asphaltene-containing toluene solution through a fixed catalyst bed (supported by glass wool) and a spectrophotometer. The decrease in absorbance at 550 nm is measured as a function of time.
There are several drawbacks to this method. The first is that the flow of a solution over a fixed catalyst bed will cause mainly chromatographic aspects i.e. adsorption effects such as adsorption capacity and competitive adsorption to be measured, rather than penetration, diffusion or accessibility effects. Although these effects will all influence the breakthrough time of the compounds, they cannot be separated and individually determined by this method. As will be explained later in this specification, determination of the diffusion effects is a prerequisite for correlation of the test results to the application conditions of the porous materials. Secondly, toluene bubbles are formed by the forced flow of toluene through the catalyst bed and the supporting glass wool. To prevent these bubbles from causing erratic reading of the absorbance by the spectrophotometer, a reservoir is needed to trap them. Further, the results of this method are only correlated to the uptake of vanadium-containing compounds by the catalyst. The publication of Saint-Just does not provide a method where the results are correlated to the accessibility of the catalyst under conditions of use.
X. Yang et al., Am. Chem. Soc., Div. Fuel Chem., Vol. 41, 1996, pp. 1013-1019, disclose a study on the hindered diffusion of asphaltenes in NiMo/Al2O3 catalysts, i.e. hydroprocessing catalysts. To this end, catalyst particles were added to a stirred vessel containing a solution of asphaltenes in THF. Periodically, samples were taken manually and the asphaltene concentration in these samples was measured by size exclusion chromatography (SEC). This publication does not provide a method where the accessibility of the catalyst is tested and correlated to the catalyst""s accessibility under conditions of use.
In one embodiment, the present invention comprises a method for testing the accessibility of porous materials with regard to compounds comprising large molecules by:
a) adding the porous material to a stirred vessel containing probe molecules dissolved in a solvent,
b) periodically analyzing the concentration of the probe molecule in the solution, and thereby measuring the probe molecule uptake, and
c) correlating the uptake to the accessibility of the porous material.
In a second embodiment, the present invention comprises an apparatus for performing the above method comprising a vessel provided with stirring means, a detector, a pump, and tubes connecting these elements.
In a third embodiment, the present invention is a method for screening catalysts comprising aging a fresh catalyst followed by quantifying the Akzo Accessibility Index according to a method wherein the probe molecule uptake is correlated to the accessibility of the porous material by plotting the relative concentration of the probe molecule against the square root of time, with the Akzo Accessibility Index (AAI) being calculated from the initial slope of this plot.
Other embodiments of the invention comprise details relating to various steps in the method and elements of the apparatus, all of which are described in detail herein.